Annex 8 Qualification of Balances
Annex 8 Qualification of Balances
Annex 8 Qualification of Balances
PA/PH/OMCL (12) 77 7R
QUALIFICATION OF EQUIPMENT
ANNEX 8: QUALIFICATION OF BALANCES
Legislative basis
QUALIFICATION OF BALANCES
1. INTRODUCTION
This document is the 8th Annex to the core document “Qualification of Equipment”, which
together should be used when planning, performing and documenting the qualification
process of balances.
The core document contains the introduction and general forms for Level I and II of
qualification, which are common to all types of instruments.
Annex 8 contains instrument-related recommendations on parameters to be checked at Level
III and IV of qualification and the corresponding typical acceptance limits, as well as practical
examples on the methodology that can be used to carry out these checks.
This guideline describes the requirements for balances (electronic - digital) used in chemical
and biological tests in OMCLs.
The following types of balances are considered in this guideline (Table 1):
Table 1
Type Ordinary name Number of digits after Accuracy
decimal position (g) Class
1. Ultra Micro Balances 7 I
2. Micro Balances 6 I
3. Semi-micro Balances 5 I
4. Analytical Balances 4 I
5. Precision Balances 1 to 3 II
6. Technical Balances 0 to 1 III
The classifications are based on the OIML R 76-1 International Recommendation document
(see Table 2).
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Table 2: The verification scale interval, number of verification scale intervals and
minimum capacity in relation to the accuracy class of an instrument.
* It is not normally feasible to test and verify an instrument to e < 1 mg due to the uncertainty of the test loads.
** For an instrument of class I with d < 0.1 mg, n may be less than 50000. The minimum capacity is reduced to 5 e for grading
instruments, i.e. instruments that determine a transport tariff or toll (e.g. postal scales and instruments weighing waste material).
On multiple range instruments, the verification scale intervals are e1, e2, …, er with e1 < e2 < … < er. Similar sub-scripts are also
used with the terms Min, n and Max.
On multiple range instruments, each range is treated as if it were an instrument with one range.
For special applications that are clearly marked on the instrument, an instrument may have weighing ranges in classes I and II, or
in classes II and III. The instrument as a whole shall then comply with the more severe requirements of 3.9 Variations due to
influence quantities and time applicable to either of the two classes (see OIML R76-1 International Recommendation document)
It is recommended that balances of types 1 to 6 (see Table 1 above) are located in a specially
designed weighing room that is vibration-proof, dust-free and has an anti-static floor. For
substances that are sensible to static electricity it is also possible to use an ionising unit.
If a balance of type 1 to 6 is located in a laboratory outside of the weighing room, then the
weighing bench on which it is placed should be in a separate part of the laboratory that is
vibration-proof and dust-free. For balances of types 1 to 4, a stable bench should be used.
It should be ensured that the weighing bench is stable, no matter from what material it is
made. The weighing bench should not deform when work is carried out on it and it must be
vibration-proof or the transfer of vibrations must not influence the weighing process. If there
is a risk of instability, the balance should be equipped with a stability indicator so that the
weight is only registered or printed after stabilisation of the balance.
The weighing bench should be non-magnetic (i.e. no steel plates) and protected against
electrostatic charges (no plastic or glass).
Temperature
As weighing results are influenced by temperature, OMCLs should ensure a constant
temperature is maintained in weighing rooms/laboratories. The typical drift of balances of
types 1 and 2 is 1-2 ppm/°C. The deviation should not exceed more than 5 °C per hour.
Atmospheric humidity
The optimum relative humidity (% RH) during a weighing process is between 40 % to 60 %
for balances of class I and II (see Table 1). The relative humidity may be expanded to 20 % to
80 % in cases where the accuracy and linearity of measurements are not affected.
Light
Balances should be protected from direct sunlight (heat).
Air
OMCLs should not place balances in the airflow of air conditioners or devices with
ventilators (such as computers or large laboratory equipment), next to doorways or in areas of
high traffic. This is because in addition to the potential temperature drift, strong air currents
can interfere with the functioning of balances. If a balance is placed in a laminar flow
workstation, e. g. for weighing of toxic material, the cabinet should be suitable for the
intended use.
Weighing vessel
OMCLs must ensure that:
- if the weighing vessel has been removed from a drying oven or dishwasher, it should
be cooled to room temperature prior to being placed on the balance.
- depending on the type of balance, it is recommended to use cotton gloves.
- no magnets should be placed on the balance or into the weighing vessel.
- be clean and, if necessary, wiped with ethanol. In the latter case a waiting time must be
taken into account before the weight is used
- be stored in a dust-free environment.
- be handled with care; the use of cotton gloves or forceps/tweezers is especially
recommended.
- have a suitable accuracy, depending on the type of balance.
- be calibrated by an accredited external service provider.
- be re-calibrated periodically.
- be clearly marked after each calibration.
- not be made of magnetisable material.
- the verification (in use control, see Table 3) may be done with non-calibrated weights.
6. FREQUENCY OF QUALIFICATION
The frequency of qualification/calibration depends on the extent of use of the balances and is
at the discretion of individual OMCLs.
Table 3 prescribes the parameters, typical frequencies and tolerance limits for the
qualification of balances for periodic and motivated instrument checks (Level III) and in-use
instrument checks (Level IV)
PA/PH/OMCL (12) 77 7R 6/11
Table 3
The following qualification tests may also be performed in addition to those described in
Table 3 (recommended, not obligatory):
7. QUALIFICATION PROCEDURE
Verification
Verification of the balance is performed by placing a suitable weight (depending on the type
of balance) in the centre of the weighing pan once and comparing the result with pre-defined
acceptance criteria. The same weight should always be used in these verifications.
Accuracy
The accuracy of the balance is checked by weighing at least three different certified weights
that cover the usual weighing range of the balance. It is recommended that the weights have
approximately 5%, 50% and 100% of the maximum capacity of the balance (or of the
maximum weight used on the balance), depending on the type of balance. It is recommended
that the weighing is repeated at least 5 times for every weight, particularly, when the results
shall also be used in the test for precision.
Linearity
The results obtained from a series of accuracy checks can be used to calculate the correlation
coefficient and to check for linearity. The correlation coefficient is calculated by comparing
the nominal and measured masses of the weights.
The acceptance criteria for the correlation coefficient shall be defined by each individual
OMCL.
(Proposed criterion: k = 1±0.0001).
Precision
The precision of the balance should be verified by weighing at least 5 times a weight that is
equivalent to approximately 50% of the maximum capacity of the balance. It is recommended
to repeat the test with a weight that is equivalent to approximately 5% of the maximum
capacity of the balance, if the balance is used at the lower range.
Eccentricity
The eccentricity test should be carried out using a weight equivalent to at least 30% of the
maximum capacity of the balance (or of the maximum weight used on the balance). The
weight should be placed between halfway to ¾ of the distance from the centre of the pan to its
edge and be measured at each location in the following sequence: centre, front left, back left,
back right, front right, and (optional) again centre.
Linearity error
Linearity error is tested at least 3 times using four weights of defined masses whose aggregate
total mass is approximately equal to half of the maximum capacity, depending on the type of
balance.
First, the combined mass of all four weights is weighed and recorded. Then, two sub-sets of
weights are made and their masses are recorded.
The linearity error of the balance is an absolute value calculated by the difference between the
combined mass of all four weights and the sum of the masses from the two sub-sets of
weights, divided by 2.
Drift test
A drift test is only performed on 5, 6 or 7 decimal scale balances. It is carried out by repeated
measurements (every 5 minutes for 30 minutes) of a control weight (depending on the type of
balance) in the morning and afternoon.
The mean measurement, standard deviation and relative standard deviation can be calculated
from the resulting data. The drift can also be extracted from the trend analysis of the
verification test (Table 3).
Minimum weight
The minimum weight value depends on the type of balance. It is determined from technical
data and the external calibration certificate.
Measurement uncertainty
The measurement uncertainty can be calculated according to GUM or other relevant
documents or can be determined from the external calibration certificate.
A simplified procedure is proposed as follows:
A weight of approximately 50% of the maximum capacity of the balance (depending on the
type of balance) is weighed at least 10 times and the mean measurement, standard deviation
and relative standard deviation are calculated.
Alternatively to the above described qualification procedures the OMCL may use the guide
OIML R76-1 from the International Organisation of Legal Metrology (current edition, see
point 10. References).
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8. QUALIFICATION REPORT
After the qualification procedure, OMCLs should appropriately record the results.
The following minimum information should be included:
Note:
If an OMCL requests qualification/calibration from an external company, it must be ensured
that this minimal information is contained in the external calibration report/certificate. The
responsible person in the OMCL should evaluate and approve this report/certificate as a
release for use of the balances.
9. GLOSSARY
Accuracy
Accuracy is the degree of closeness of a measurement to the true value of the quantity being
measured.
Calibration
Calibration is a demonstration that an instrument or a device produces results within specified
limits when compared to those produced by a standard (or a reference standard that is
traceable to a national or international standard) over an appropriate range of measurements.
Calibration is a determination of the deviation between measurements and the true value
under specified measuring conditions.
Eccentricity
Eccentricity is an error relating to variations in the positioning of weights on the weighing
pan.
PA/PH/OMCL (12) 77 7R 10/11
Levelling
Levelling is a procedure to ensure the balance is in the horizontal position.
Linearity
Linearity refers to the ability to deliver identical sensitivity throughout the weighing capacity
of a balance.
Linearity error
Linearity error is the difference between the digital display for a weight that weighs 50% of
the full weighing capacity of the instrument and its true mass.
Measurement uncertainty
Measurement uncertainty is a parameter, associated with the result of a measurement, that
characterises the dispersion of the values that could reasonably be attributed to the
measurement variable (VIM 3.9).
Measurement uncertainty is generally expressed by the standard uncertainty u or the
expanded measurement uncertainty U (confidence interval). There are several ways of
calculating the measurement uncertainty, one of them is given in the GUM.
Precision
Precision is a measure of the reproducibility of results from independent measurements.
Qualification
See OMCL Guideline “Qualification of equipment – Core document”, PA/PH/OMCL (08) 73
2R.
Weights
Weights are objects, regulated in regard to their physical and metrological characteristics such
as shape, dimensions, material, surface quality, nominal value, density, magnetic properties
and maximum permissible error.
11/11 OMCL (12) 77 7R
10. REFERENCES
ILAC G24 2007 (OIML D10 2007) GUIDELINES FOR THE DETERMINATION OF
CALIBRATION INTERVALS OF MEASURING INSTRUMENTS
ISO 9001
Internet pages:
http://www.msl.irl.cri.nz/sites/all/files/training-manuals/tg25-december2-2010.pdf
http://gmponblog.vinvarun.biz/2008/08/sop-calibration-of-balances.html